02 20 code maintenance

General code maintenance:

• remove dead code
• remove as many #define as possible and use constexpr instead
• remove adafs occurrences and replace with gkfs
• reformat code to unify code style as of CPPCoreGuidelines
• rearrange imports, remove unused imports
• separate cmake configurations and FS configurations
• encapsulate non-prefix gkfs functions into namespaces
• change copyright statement to 2020

Since most code has been merged. This is a good time to tackle this now.